Onion Skinning and Thickness Planing/Sanding

      For accuracy and consistency, what's practical and economical: a belt sander, a planer, or a fly-cutter on the CNC? April 21, 2011

Question
We are doing multiple small parts on a CNC point to point machine. The wood is mostly pine and mahogany. We are leaving a heavy 16th relief in the kerfs and want to separate the parts accurately and efficiently. Currently we have an older Powermatic HSS that is a great work horse but isn't accurate/consistent enough for CNC prep. We’re using a micrometer for each part before placement defeats the efficiency and purpose of the CNC.

I am leaning towards a good digital thickness planer with Tersa heads as we want to be accurate pre and post placement of parts. For instance the wood needs to be exactly 1.5" ect to be put on the CNC to reference the Z axis, make the millings, and then feed it through the planer and take off the 16th and separate the parts. I like the Martin T 45 for this as I am thinking a wide belt sander will clog too quickly. Money is a concern to a degree but precision and accuracy are critical.

Forum Responses
(Solid Wood Machining Forum)
From contributor U:
Have you thought of using a calibrating sander that uses cutter heads? This way you have a planer with the size of a wide belt sander. It is a great option and these machines are accurate and finished with (1) head that is sanding to follow up after the cutters.



From the original questioner:
I am concerned about using all that horsepower for small runs.


From contributor Z:
Money can't be too much of a concern if you are looking at Martin. I would think that any good planer (digital or otherwise) should do what you are asking. You can put an Accurate Technology or Wixley Dro on any good manual planer you want to get the kind of accuracy you're after.


From contributor R:
Are you taking them to a CNC?


From contributor D:
60 grit widebelt and two passes works for me. Those belts get a long lifespan for us in hardwoods, not with pine. We sand everything prior to machining on the CNC to ensure accuracy and a smooth surface for suction pods to adhere to. I like separate machines myself, which can be used for different things, by different people, at the same time. Combination machines are nice, but in my opinion not a great timesaver unless you're really a high volume shop.


From contributor U:
After reading some more of the posts a combination machine would really depend upon the amount of work. I would also agree with that if you are running solid wood and want to guarantee a thickness. Calibrating the material with a drum belt sander prior to machining. Then you wouldn't have to worry about onion skinning. It is possible to onion skin and then can separate the parts slowly and you won't have movement. This can be accomplished on any CNC.


From the original questioner:
No inaccuracies with the machine at all. Actually the accuracy is frighteningly accurate. The problem is our planer is old and doesn't hold the tolerance over the width. This is sending inconsistent parts to the CNC as we are working face up. I am thinking two birds one stone here meaning sending an accurate piece to the machine and then separating the parts afterwards. My feeling is that the sander you describe is a lot of horsepower for start and stop production. We have a demand charge here so it could get very expensive to start up all that horsepower for low volumes. Most importantly the primary wood is Canadian pine - very sappy and will gum up 60 grit in a heartbeat so I have been told. I am open to suggestions.


From contributor U:
The horsepower would be higher, but you also wouldn't be running the calibrating sander all day. There is another option. If you have a tool changer on your router, you could use that to calibrate your parts. I would load the parts on the CNC, mill them flat and then flip them over, mill them flat on this side. Perform you machine and then your parts are done. This could be done with a 2-3 fly cutter. It is an option to save the cost of a machine and repairing your planer to bring it back into spec.



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